Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery which has a high energy density and excellent cycle characteristics. The present invention pertains to a lithium secondary battery comprising a positive electrode, a negative electrode that does not include a negative electrode active material, and an electrolyte, the electrolyte comprising a lithium salt and a cyclic fluorine-containing compound comprising a cyclic hydrocarbon skeleton substituted with at least one fluorine atom.

Description

[Technical field]

[0001] The present invention relates to a lithium secondary battery. [Background technology]

[0002] In recent years, technology for converting natural energy such as solar or wind power into electrical energy has been attracting attention. Accordingly, various secondary batteries have been developed as electricity storage devices that are highly safe and capable of storing a large amount of electrical energy.

[0003] Among them, lithium secondary batteries, which charge and discharge by transferring lithium ions between a positive electrode and a negative electrode, are known to exhibit high voltage and high energy density. A typical lithium secondary battery is the lithium ion secondary battery (LIB), which has active materials capable of retaining lithium elements in the positive electrode and the negative electrode, and charges and discharges by transferring lithium ions between the positive electrode active material and the negative electrode active material.

[0004] Furthermore, in order to achieve high energy density, lithium secondary batteries (lithium metal batteries; LMBs) have been developed that use lithium metal as the negative electrode active material instead of materials that can insert lithium ions, such as carbon materials. For example, Patent Document 1 discloses a rechargeable battery that uses a lithium metal-based electrode as the negative electrode.

[0005] In addition, for the purpose of further increasing the energy density and improving productivity, lithium secondary batteries using negative electrodes that do not have negative electrode active materials such as carbon materials and lithium metal have been developed. For example, Patent Document 2 discloses a lithium secondary battery including a positive electrode, a negative electrode, a separator and an electrolyte interposed between them, in which metal particles are formed on a negative electrode current collector, and are transferred from the positive electrode by charging to form lithium metal on the negative electrode current collector in the negative electrode. Patent Document 2 discloses that such a lithium secondary battery can provide a lithium secondary battery with improved performance and life by solving problems caused by the reactivity of lithium metal and problems that occur during the assembly process. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2006-500755 [Patent Document 2] Special Publication No. 2019-505971 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the present inventors have conducted detailed studies on conventional batteries, including those described in the above patent documents, and have found that at least one of the energy density and cycle characteristics is insufficient.

[0008] For example, it is difficult to sufficiently increase the energy density of a lithium secondary battery equipped with a negative electrode having a negative electrode active material due to the volume and mass occupied by the negative electrode active material. In addition, in the case of anode-free type lithium secondary battery equipped with a negative electrode not having a negative electrode active material, dendritic lithium metal is easily formed on the surface of the negative electrode by repeated charging and discharging, and short circuits and capacity reduction are easily caused, so that the cycle characteristics are insufficient.

[0009] The present invention has been made in consideration of the above problems, and has an object to provide a lithium secondary battery having high energy density and excellent cycle characteristics. [Means for solving the problem]

[0010] A lithium secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode having no negative electrode active material, and an electrolyte, the electrolyte including a lithium salt and a cyclic fluorine compound having a cyclic hydrocarbon skeleton substituted with at least one fluorine atom.

[0011] Such lithium secondary batteries have a high energy density because they are equipped with a negative electrode that does not have a negative electrode active material, and charging and discharging are performed by depositing lithium metal on the surface of the negative electrode and electrolytic dissolution of the deposited lithium metal.

[0012] The present inventors have also found that a lithium secondary battery containing a cyclic fluorine compound having a cyclic hydrocarbon skeleton substituted with at least one fluorine atom has excellent cycle characteristics. This is thought to be due to the fact that a solid electrolyte interface layer (hereinafter also referred to as "SEI layer") is easily formed on the negative electrode surface in an embodiment containing such a cyclic fluorine compound, but the cause is not limited to this. In addition, since the SEI layer has ion conductivity, the reactivity of the lithium deposition reaction on the negative electrode surface on which the SEI layer is formed tends to be uniform in the planar direction of the negative electrode surface. Furthermore, the present inventors have found that in this embodiment, the compatibility between the solvents in the electrolyte is improved, and the electrolyte maintains a stable composition even when the battery is repeatedly charged and discharged. This is believed to be because cyclic fluorine compounds tend to have a higher polarity than chain fluorine compounds and tend to be uniformly distributed in the electrolyte. It is believed that the use of an electrolyte in which the cyclic fluorine compounds are uniformly distributed further improves the cycle characteristics of the battery due to, for example, the factors described above. Therefore, in the lithium secondary battery, even when repeatedly charged and discharged, the growth of dendritic lithium metal on the negative electrode is suppressed, and the battery has excellent cycle characteristics. The factors that facilitate the formation of an SEI layer by including a fluorine compound and that improve compatibility by including a cyclic fluorine compound are not particularly limited.

[0013] Furthermore, the present inventors have found that by including the cyclic fluorine compound in the electrolyte of a lithium secondary battery, the volume expansion rate of the battery during repeated charging and discharging is reduced. It is presumed that cyclic fluorine compounds tend to have higher polarity than chain fluorine compounds, and therefore have higher boiling points and lower vapor pressures. Therefore, the lithium secondary battery of the present invention tends to have a suppressed volume expansion rate of the battery and is also excellent in safety.

[0014] In the lithium secondary battery according to one embodiment of the present invention, the cyclic fluorine compound preferably has polarity. According to such an embodiment, the compatibility between the solvents in the electrolyte is further improved, and the volume expansion rate of the battery is further suppressed, so that the lithium secondary battery tends to have more excellent cycle characteristics and safety.

[0015] In the lithium secondary battery according to one embodiment of the present invention, preferably, in the cyclic fluorine compound, the ratio (F / (F+H)) of the number of fluorine atoms (F) to the total number of fluorine atoms and hydrogen atoms (F+H) is 0.20 or more and 1.0 or less. According to such an embodiment, the properties of the SEI layer tend to be favorable, and therefore the lithium secondary battery tends to have further excellent cycle characteristics.

[0016] In the lithium secondary battery according to one embodiment of the present invention, the number of carbon atoms constituting the cyclic hydrocarbon skeleton is preferably from 4 to 15. According to such an embodiment, the compatibility in the electrolyte is further improved and the volume expansion rate of the battery is further likely to be suppressed, so that the lithium secondary battery tends to have more excellent cycle characteristics and safety.

[0017] In the lithium secondary battery according to one embodiment of the present invention, the electrolyte preferably contains LiN(SO2F)2 as a lithium salt. According to such an embodiment, the cycle characteristics of the lithium secondary battery tend to be further improved.

[0018] In the lithium secondary battery according to one embodiment of the present invention, the content of the cyclic fluorine compound is preferably 10% by volume or more and 90% by volume or less with respect to the total amount of the solvent components of the electrolyte solution. According to such an embodiment, the lithium secondary battery tends to have further excellent cycle characteristics and safety.

[0019] In the lithium secondary battery according to one embodiment of the present invention, the electrolyte preferably further contains an ether compound having no fluorine atom. According to such an embodiment, the lithium secondary battery has improved solubility of the lithium salt in the electrolyte, and therefore tends to have improved ionic conductivity in the electrolyte and excellent cycle characteristics.

[0020] In the lithium secondary battery according to one embodiment of the present invention, the ether compound is preferably a compound containing 2 to 5 ether bonds. According to such an embodiment, the solubility of the electrolyte in the electrolytic solution is improved, and the cycle characteristics of the lithium secondary battery tend to be further improved.

[0021] In the lithium secondary battery according to one embodiment of the present invention, the content of the ether compound is preferably 10% by volume or more and 70% by volume or less with respect to the total amount of the solvent components of the electrolyte solution. According to such an embodiment, the solubility of the electrolyte in the electrolyte solution is improved, and the cycle characteristics of the lithium secondary battery tend to be further improved.

[0022] In the lithium secondary battery according to one embodiment of the present invention, the electrolyte preferably further contains a chain fluorine compound having at least one of the monovalent groups represented by the following formula (A) or formula (B). According to such an embodiment, the lithium secondary battery tends to have further excellent cycle characteristics. [ka] [ka] In the above formula, the wavy line represents a bonding site on the monovalent group.

[0023] In the lithium secondary battery according to one embodiment of the present invention, the amount of the chain fluorine compound is preferably 10% by volume or more and 85% by volume or less with respect to the total amount of the solvent components of the electrolyte solution. According to such an embodiment, the lithium secondary battery tends to have further excellent cycle characteristics.

[0024] In the lithium secondary battery according to one embodiment of the present invention, the cyclic hydrocarbon skeleton is preferably a saturated cyclic hydrocarbon skeleton. According to such an embodiment, the lithium secondary battery tends to have more excellent cycle characteristics.

[0025] In the lithium secondary battery according to one embodiment of the present invention, preferably, in the cyclic fluorine compound, the ratio (F / (F+H)) of the number of fluorine atoms (F) to the total number of fluorine atoms and hydrogen atoms (F+H) is 0.7 or more and 1.0 or less, and the number of carbon atoms constituting the cyclic hydrocarbon skeleton is 5 or 6. According to such an embodiment, the lithium secondary battery tends to be more excellent in cycle characteristics and safety. Effect of the Invention

[0026] According to the present invention, it is possible to provide a lithium secondary battery having high energy density and excellent cycle characteristics. [Brief description of the drawings]

[0027] [Figure 1] 1 is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings as necessary. In the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted. Furthermore, unless otherwise specified, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0029] [Present embodiment] (lithium secondary battery) Fig. 1 is a schematic cross-sectional view of a lithium secondary battery according to this embodiment. As shown in Fig. 1, the lithium secondary battery 100 of this embodiment includes a positive electrode 120, a negative electrode 140 that does not have a negative electrode active material, a separator 130 disposed between the positive electrode 120 and the negative electrode 140, and an electrolyte not shown in Fig. 1. The positive electrode 120 has a positive electrode current collector 110 on the surface opposite to the surface facing the separator 130. Each component of the lithium secondary battery 100 will be described below.

[0030] (Negative electrode) The negative electrode 140 does not have a negative electrode active material. In this specification, the term "negative electrode active material" refers to a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, in the negative electrode. Specifically, the negative electrode active material of this embodiment includes a host material of lithium metal and lithium element (lithium ion or lithium metal). The host material of lithium element means a material provided to hold lithium ion or lithium metal in the negative electrode. The mechanism of such holding is not particularly limited, but includes, for example, intercalation, alloying, and occlusion of metal clusters, and is typically intercalation.

[0031] In the lithium secondary battery of this embodiment, since the negative electrode does not have a negative electrode active material before the initial charge of the battery, lithium metal is deposited on the negative electrode, and the deposited lithium metal is electrolytically dissolved to perform charging and discharging. Therefore, in the lithium secondary battery of this embodiment, the volume occupied by the negative electrode active material and the mass of the negative electrode active material are reduced compared to a lithium secondary battery having a negative electrode active material, and the volume and mass of the entire battery are reduced, so that the energy density is high in principle.

[0032] In the lithium secondary battery 100 of this embodiment, the negative electrode 140 does not have a negative electrode active material before the initial charge of the battery, lithium metal is deposited on the negative electrode by charging the battery, and the deposited lithium metal is electrolytically dissolved by discharging the battery. Therefore, in the lithium secondary battery of this embodiment, the negative electrode functions as a negative electrode current collector.

[0033] The lithium secondary battery 100 of this embodiment differs from a lithium ion battery (LIB) and a lithium metal battery (LMB) in the following respects. In a lithium ion battery (LIB), the negative electrode has a host material of lithium element (lithium ion or lithium metal), and when the battery is charged, the material is filled with lithium element, and when the host material releases the lithium element, the battery is discharged. The LIB differs from the lithium secondary battery 100 of this embodiment in that the negative electrode has a host material of lithium element. A lithium metal battery (LMB) is manufactured using an electrode having lithium metal on its surface or elemental lithium metal as the negative electrode. That is, the LMB differs from the lithium secondary battery 100 of this embodiment in that the negative electrode has lithium metal as the negative electrode active material immediately after the battery is assembled, i.e., before the initial charge of the battery. The LMB uses an electrode containing highly flammable and reactive lithium metal in its manufacture, but the lithium secondary battery 100 of this embodiment uses a negative electrode that does not have lithium metal, and is therefore safer and more productive.

[0034] In this specification, the negative electrode "has no negative electrode active material" means that the negative electrode 140 does not have or substantially does not have a negative electrode active material. The negative electrode 140 substantially does not have a negative electrode active material means that the content of the negative electrode active material in the negative electrode 140 is 10 mass% or less with respect to the entire negative electrode. The content of the negative electrode active material in the negative electrode is preferably 5.0 mass% or less with respect to the entire negative electrode 140, and may be 1.0 mass% or less, 0.1 mass% or less, or 0.0 mass% or less. When the negative electrode 140 does not have a negative electrode active material or the content of the negative electrode active material in the negative electrode 140 is within the above range, the energy density of the lithium secondary battery 100 becomes high.

[0035] In this specification, the term "before initial charging" refers to the state of the battery from assembly to the first charging, and the term "at the end of discharging" refers to the state of the battery when the voltage is 1.0 V to 3.8 V, preferably 1.0 V to 3.0 V.

[0036] In this specification, the term "lithium secondary battery having a negative electrode that does not have a negative electrode active material" means that the negative electrode 140 does not have a negative electrode active material before the initial charge of the battery. Therefore, the phrase "negative electrode that does not have a negative electrode active material" may be rephrased as "negative electrode that does not have a negative electrode active material before the initial charge of the battery", "negative electrode that does not have a negative electrode active material other than lithium metal regardless of the charge state of the battery and does not have lithium metal before the initial charge", or "negative electrode current collector that does not have lithium metal before the initial charge", etc. In addition, the term "lithium secondary battery having a negative electrode that does not have a negative electrode active material" may be rephrased as an anode-free lithium battery, a zero anode lithium battery, or an anode-less lithium battery.

[0037] In the negative electrode 140 of this embodiment, regardless of the charged state of the battery, the content of negative electrode active material other than lithium metal is 10 mass% or less relative to the entire negative electrode, and may preferably be 5.0 mass% or less, 1.0 mass% or less, 0.1 mass% or less, 0.0 mass% or less, or even 0 mass%. Furthermore, in the negative electrode 140 of this embodiment, before initial charging, the lithium metal content is 10% by mass or less, preferably 5.0% by mass or less, 1.0% by mass or less, 0.1% by mass or less, 0.0% by mass or less, or even 0% by mass, relative to the entire negative electrode.

[0038] In the lithium secondary battery 100 of this embodiment, when the battery voltage is 1.0 V or more and 3.5 V or less, the lithium metal content may be 10 mass % or less (preferably 5.0 mass % or less, and may be 1.0 mass % or less) with respect to the entire negative electrode 140; when the battery voltage is 1.0 V or more and 3.0 V or less, the lithium metal content may be 10 mass % or less (preferably 5.0 mass % or less, and may be 1.0 mass % or less) with respect to the entire negative electrode 140; or when the battery voltage is 1.0 V or more and 2.5 V or less, the lithium metal content may be 10 mass % or less (preferably 5.0 mass % or less, and may be 1.0 mass % or less) with respect to the entire negative electrode 140.

[0039] In the lithium secondary battery 100 of this embodiment, the mass M of lithium metal deposited on the negative electrode when the battery voltage is 4.2 V is 4.2 The mass M of lithium metal deposited on the negative electrode when the battery voltage is 3.0 V 3.0 Ratio of M 3.0 / M 4.2 is preferably 40% or less, more preferably 38% or less, and further preferably 35% or less. 3.0 / M 4.2 may be 1.0% or more, 2.0% or more, 3.0% or more, or 4.0% or more.

[0040] Examples of the negative electrode active material of this embodiment include lithium metal and alloys containing lithium metal, carbon-based materials, metal oxides, and metals that are alloyed with lithium and alloys containing the metals. The carbon-based materials are not particularly limited, but include graphene, graphite, hard carbon, mesoporous carbon, carbon nanotubes, and carbon nanohorns. The metal oxides are not particularly limited, but include titanium oxide compounds, tin oxide compounds, and cobalt oxide compounds. The metals that are alloyed with lithium include silicon, germanium, tin, lead, aluminum, and gallium.

[0041] The negative electrode 140 of this embodiment is not particularly limited as long as it does not have a negative electrode active material and can be used as a current collector. For example, at least one selected from the group consisting of Cu, Ni, Ti, Fe, and other metals that do not react with Li, and alloys thereof, and stainless steel (SUS), and preferably at least one selected from the group consisting of Cu, Ni, alloys thereof, and stainless steel (SUS). When such a negative electrode is used, the energy density and productivity of the battery tend to be further improved. When SUS is used for the negative electrode, various types of SUS that have been known in the past can be used. The above-mentioned negative electrode materials are used alone or in combination of two or more types. In this specification, the term "metal that does not react with Li" means a metal that does not react with lithium ions or lithium metal to form an alloy under the operating conditions of a lithium secondary battery.

[0042] The capacity of the negative electrode 140 is sufficiently smaller than the capacity of the positive electrode 120, and may be, for example, 20% or less, 15% or less, 10% or less, or 5% or less. The capacities of the positive electrode 120 and the negative electrode 140 can be measured by a conventionally known method.

[0043] The average thickness of the negative electrode 140 is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and further preferably 6 μm or more and 15 μm or less. According to such an embodiment, the volume occupied by the negative electrode 140 in the lithium secondary battery 100 is reduced, and therefore the energy density of the lithium secondary battery 100 is further improved.

[0044] In this embodiment, the thickness can be measured by a known measurement method. For example, the thickness can be measured by cutting the lithium secondary battery in the thickness direction and observing the exposed cut surface with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The "average thickness" and "thickness" in this embodiment are obtained by calculating the arithmetic mean of measurements taken three or more times, preferably five or more times.

[0045] (electrolyte) The electrolyte is a solution containing an electrolyte and a solvent, has ion conductivity, and acts as a conductive path for lithium ions. The electrolyte may be impregnated into the separator 130, or may be enclosed in a sealed container together with the laminate of the positive electrode 120, the separator 130, and the negative electrode 140.

[0046] The electrolyte solution of the present embodiment contains a lithium salt and a cyclic fluorine compound having a cyclic hydrocarbon skeleton substituted with at least one fluorine atom (hereinafter, also simply referred to as a "cyclic fluorine compound").

[0047] In general, in an anode-free lithium secondary battery having an electrolyte, a solvent in the electrolyte is decomposed to form an SEI layer on the surface of the negative electrode. The SEI layer suppresses further decomposition of the components in the electrolyte in the lithium secondary battery, as well as irreversible reduction of lithium ions and generation of gas caused thereby. In addition, since the SEI layer has ion conductivity, the reactivity of the lithium deposition reaction becomes uniform in the plane direction of the negative electrode surface on the surface of the negative electrode on which the SEI layer is formed. Therefore, promoting the formation of the SEI layer is very important for improving the performance of an anode-free lithium secondary battery. The present inventors have found that in a lithium secondary battery containing the above-mentioned cyclic fluorine compound as a solvent, a good quality SEI layer is easily formed on the surface of the negative electrode, and the growth of dendritic lithium metal on the negative electrode is suppressed, resulting in improved cycle characteristics. The reason for this is not necessarily clear, but the following factors are considered to be the cause.

[0048] During charging of the lithium secondary battery 100, particularly during initial charging, it is believed that not only lithium ions but also the cyclic fluorine compound, which is the solvent, is reduced on the negative electrode. It is presumed that the cyclic fluorine compound is likely to undergo reactions such as detachment of particularly weakly bonded parts in the cyclic hydrocarbon skeleton due to the fact that part of the structure of the cyclic fluorine compound is substituted with fluorine. As a result, during charging of the lithium secondary battery 100, a part or all of the cyclic fluorine compound is adsorbed onto the surface of the negative electrode, and an SEI layer is generated starting from the adsorbed part, so that it is presumed that the lithium secondary battery 100 is likely to form an SEI layer. However, the cause is not limited to the above.

[0049] Furthermore, it has been found that cyclic fluorine compounds have a cyclic hydrocarbon skeleton substituted with at least one fluorine atom, and thus have high compatibility with other solvents in the electrolyte. This is believed to be due to the fact that cyclic fluorine compounds have a more limited conformation of the molecular structure than chain fluorine compounds, and therefore exhibit a greater polarity. However, the reason for this is not limited to this. Therefore, it is believed that the above cyclic fluorine compounds are uniformly distributed in the electrolyte solution, forming a more uniform SEI layer.

[0050] As described above, cyclic fluorine compounds tend to have a larger polarity than chain fluorine compounds, and therefore tend to have stronger intermolecular interactions and higher boiling points. The electrolyte solution containing such cyclic fluorine compounds tends to have a lower vapor pressure, and therefore the lithium secondary battery of the present embodiment tends to have a lower volume expansion rate and superior safety even when repeatedly charged and discharged.

[0051] In this specification, the term "cyclic hydrocarbon skeleton" refers to a structure in which a hydrocarbon chain is cyclized, formed by bonding multiple carbon atoms in a ring, and can also be referred to as a cyclic hydrocarbon skeleton. In addition, in the cyclic hydrocarbon skeleton, hydrogen atoms directly bonded to the carbon atoms bonded in a ring may be substituted with any substituent. That is, the cyclic hydrocarbon skeleton is composed of a ring structure made of multiple carbon atoms and hydrogen atoms and / or substituents bonded to the ring structure. In the cyclic hydrocarbon skeleton, all hydrogen atoms may be substituted with substituents. As used herein, the term "fluorine compound" refers to a compound having at least one fluorine atom.

[0052] The electrolyte solution of the present embodiment typically contains a cyclic fluorine compound as a solvent. That is, in the usage environment of the lithium secondary battery, the cyclic fluorine compound can dissolve the electrolyte to prepare an electrolyte solution in a solution phase, or the compound alone or a mixture with other compounds is preferably in a liquid state.

[0053] The cyclic fluorine compound contained in the electrolyte solution of the present embodiment is preferably polar. The polarity of the cyclic fluorine compound improves the compatibility between the solvents in the electrolyte solution, and the SEI layer is uniformly formed. As a result, the lithium metal can be prevented from growing in a dendritic form on the negative electrode surface, and the lithium secondary battery tends to have excellent cycle characteristics. In this specification, a compound having "polarity" means that the center of gravity of the positive charge and the center of gravity of the negative charge do not coincide in the molecular structure of the compound. In other words, if the dipole moment of the entire molecule is not 0, it can be said to have polarity.

[0054] The cyclic fluorine compound contained in the electrolyte of the present embodiment has a cyclic hydrocarbon skeleton substituted with at least one fluorine atom. In the cyclic fluorine compound, the ratio (F / (F+H)) of the number of fluorine atoms (F) to the total number (F+H) of fluorine atoms and hydrogen atoms is not particularly limited, but is, for example, 0.10 or more and 1.0 or less. From the viewpoint of improving the cycle characteristics and / or stability of the battery, the ratio (F / (F+H)) is preferably 0.20 or more, 0.25 or more, 0.30 or more, 0.35 or more, 0.40 or more, 0.45 or more, or 0.50 or more. From the same viewpoint, the ratio (F / (F+H)) is preferably less than 1.0, 0.95 or less, 0.90 or less, 0.85 or less, or 0.80 or less. The ratio (F / (F+H)) is particularly preferably, for example, 0.7 or more and 1.0 or less.

[0055] The number of fluorine atoms in the cyclic fluorine compound is not particularly limited as long as it is 1 or more, and is, for example, 1 to 45. From the viewpoint of improving the cycle characteristics and / or stability of the battery, the number of fluorine atoms in the cyclic fluorine compound is preferably 3 or more, 5 or more, 6 or more, or 7 or more. From the same viewpoint, the number is preferably 40 or less, 35 or less, 30 or less, 25 or less, 20 or less, 15 or less, or 10 or less.

[0056] In the cyclic fluorine compound of this embodiment, the number of carbon atoms constituting the cyclic hydrocarbon skeleton (i.e., constituting the ring) is not particularly limited, but is, for example, 3 or more and 20 or less. From the viewpoint of further improving cycle characteristics and safety, the number of carbon atoms constituting the cyclic hydrocarbon skeleton is preferably 4 or more, 5 or more, or 6 or more. From the same viewpoint, the number of carbon atoms is preferably 18 or less, 15 or less, 12 or less, 10 or less, or 8 or less. The number of carbon atoms is particularly preferably 5 or 6. When the cyclic hydrocarbon skeleton has a skeleton in which two or more ring structures such as a bicyclo ring are fused, the number of carbon atoms constituting each ring is preferably 3 or more and 10 or less.

[0057] The number of carbon atoms constituting the cyclic fluorine compound of the present embodiment is not particularly limited, and is, for example, from 4 to 30. In addition, the number of carbon atoms constituting the cyclic fluorine compound may be from 5 to 25, from 6 to 20, from 7 to 15, or from 8 to 12.

[0058] In the cyclic fluorine compound of the present embodiment, the cyclic hydrocarbon skeleton may be a saturated cyclic hydrocarbon skeleton or an unsaturated cyclic hydrocarbon skeleton. From the viewpoint of improving the cycle characteristics of the battery, the cyclic fluorine compound preferably has a saturated cyclic hydrocarbon skeleton. The term "saturated cyclic hydrocarbon skeleton" means that all bonds between carbon atoms constituting the cyclic hydrocarbon skeleton are single bonds. The term "unsaturated cyclic hydrocarbon skeleton" means that the bonds between carbon atoms constituting the cyclic hydrocarbon skeleton have at least one bond that is not a single bond (double bond or triple bond). From the same viewpoint, the cyclic fluorine compound of the present embodiment preferably does not have a carbon-carbon double bond, and more preferably does not have a double bond.

[0059] In the cyclic fluorine compound of this embodiment, the cyclic hydrocarbon skeleton may have a substituent other than a fluorine atom. Such a substituent may be an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, a cycloalkenyl group, a cycloalkynyl group, and an aryl group, each of which may have a further substituent; a nitrile group; a halogen group; a silyl group; a hydroxy group; an alkoxy group which may have a further substituent; and an aryloxy group which may have a further substituent. Examples of the aryl group include a phenyl group, a naphthyl group, a furyl group, a thienyl group, and a pyridyl group. When the substituent has a hydrocarbon chain, the hydrocarbon chain may be linear or branched. Substituents that the above alkyl group, alkenyl group, alkynyl group, cycloalkyl group, cycloalkenyl group, cycloalkynyl group, aryl group, alkoxy group, and aryloxy group may further have include a nitrile group, a halogen group, a silyl group, and a hydroxy group. The number of substituents other than fluorine atoms bonded to the cyclic hydrocarbon skeleton of the cyclic fluorine compound is not particularly limited, and may be, for example, from 1 to 10. The number of substituents other than fluorine atoms may be 8 or less, 5 or less, 4 or less, 3 or less, or 2 or less within the above range.

[0060] When the substituent bonded to the cyclic hydrocarbon skeleton has a carbon atom, the number of carbon atoms of the substituent is preferably 1 to 10, more preferably 1 to 5 or 1 to 3. The substituent bonded to the cyclic hydrocarbon skeleton is preferably selected from an alkyl group, an alkenyl group, and an alkynyl group having 1 to 5 carbon atoms (preferably 1 to 3) which may have a substituent. The substituent bonded to the cyclic hydrocarbon skeleton is more preferably selected from an alkyl group, an alkenyl group, and an alkynyl group having 1 to 5 carbon atoms (preferably 1 to 3) which may have a halogen group (preferably a fluorine atom). The substituent bonded to the cyclic hydrocarbon skeleton is further preferably an alkyl group having 1 to 3 carbon atoms which may be substituted by a fluorine atom.

[0061] The substituent bonded to the cyclic hydrocarbon skeleton is preferably substituted with a fluorine atom. The substituent substituted with a fluorine atom is not particularly limited, but may be, for example, a trifluoromethyl group, a tetrafluoroethyl group, a perfluoroethyl group, a tetrafluoropropyl group, a perfluorobutyl group, a perfluorohexyl group, a perfluorooctyl group, a perfluorodecyl group, a fluoromethoxy group, a fluoroethoxy group, a fluoropropoxy group, and a fluorobutoxy group. Among the substituents of such fluorine compounds, from the viewpoint of further improving the cycle characteristics of a lithium secondary battery, a trifluoromethyl group, a tetrafluoroethyl group, a tetrafluoropropyl group, a fluoroethoxy group, and a fluoropropoxy group are preferred, a trifluoromethyl group, a 1,1,2,2-tetrafluoroethoxy group, a 1,1,2,2-tetrafluoropropoxy group, and a 2,2,3,3-tetrafluoropropoxy group are more preferred, and a trifluoromethyl group is even more preferred.

[0062] A preferred embodiment of the cyclic fluorine compound of this embodiment is, for example, a cyclic fluorine compound in which the ratio (F / (F+H)) of the number of fluorine atoms (F) to the total number of fluorine atoms and hydrogen atoms (F+H) is 0.7 to 1.0, and the number of carbon atoms constituting the cyclic hydrocarbon skeleton is 5 or 6. According to such an embodiment, the cycle characteristics and / or safety of the battery tend to be further improved.

[0063] The cyclic fluorine compound in this embodiment is not particularly limited, but examples thereof include 1,1,2,2,3,3,4-heptafluorocyclopentane, hexadecafluoro(1,3-dimethylcyclohexane), tetradecafluoromethylcyclohexane, fluorocyclohexane, fluorocyclopentane, octadecafluorodecahydronaphthalene, octafluorocyclobutane, octafluoronaphthalene, and perfluoroanthracene. From the viewpoint of effectively and reliably exerting the above-mentioned effects of the cyclic fluorine compound in this embodiment, the cyclic fluorine compound is preferably 1,1,2,2,3,3,4-heptafluorocyclopentane, hexadecafluoro(1,3-dimethylcyclohexane), tetradecafluoromethylcyclohexane, fluorocyclohexane, fluorocyclopentane, or octadecafluorodecahydronaphthalene, more preferably 1,1,2,2,3,3,4-heptafluorocyclopentane or hexadecafluoro(1,3-dimethylcyclohexane), or even more preferably 1,1,2,2,3,3,4-heptafluorocyclopentane. In this embodiment, the electrolyte solution needs to contain at least one type of cyclic fluorine compound, and may contain two or more types in combination.

[0064] The content of the cyclic fluorine compound in the electrolyte of the present embodiment is not particularly limited, but is, for example, 1.0 volume % or more and 95 volume % or less with respect to the total amount of the solvent components of the electrolyte. From the viewpoint of effectively and reliably exerting the above-mentioned effect of the cyclic fluorine compound in the lithium secondary battery, the content of the cyclic fluorine compound is preferably 5.0 volume % or more, 10 volume % or more, or 15 volume % or more with respect to the total amount of the solvent components of the electrolyte, and may be 20 volume % or more, 25 volume % or more, 30 volume % or more, or 50 volume % or more. From the same viewpoint, the content of the cyclic fluorine compound is preferably 90 volume % or less, or 85 volume % or less with respect to the total amount of the solvent components of the electrolyte, and may be 80 volume % or less, 70 volume % or less, 60 volume % or less, or 55 volume % or less.

[0065] The electrolytic solution of the present embodiment may contain a fluorine compound other than the cyclic fluorine compound as a solvent. For example, such a fluorine compound may be a chain fluorine compound. From the viewpoint of further improving the cycle characteristics of the lithium secondary battery, the electrolytic solution of the present embodiment preferably contains a chain fluorine compound. From the viewpoint of the stability of the lithium secondary battery, the electrolytic solution of the present embodiment preferably contains only the cyclic fluorine compound as a solvent having fluorine atoms. The electrolytic solution of the present embodiment may contain only the cyclic fluorine compound and the first chain fluorine compound described later as a solvent having fluorine atoms.

[0066] Of such chain-like fluorine compounds, the electrolyte solution of the present embodiment preferably further contains a chain-like fluorine compound having at least one of the monovalent groups represented by the following formula (A) or formula (B) (hereinafter also referred to as a "first chain-like fluorine compound"). By containing the first chain-like fluorine compound, an SEI layer is more likely to be formed during charging of the lithium secondary battery, particularly during initial charging, and the lithium secondary battery tends to have excellent cycle characteristics and / or rate characteristics. [ka] [ka] In the above formula, the wavy line represents a bonding site in a monovalent group.

[0067] The first chain fluorine compound in this embodiment includes a compound containing both the structures represented by the above formula (A) and the above formula (B), a compound containing the structure represented by the above formula (A) and not containing the structure represented by the above formula (B), and a compound not containing the structure represented by the above formula (A) and containing the structure represented by the above formula (B). The electrolyte solution of the present embodiment may contain a chain-like fluorine compound other than the first chain-like fluorine compound described above. That is, the electrolyte solution of the present embodiment may contain a chain-like fluorine compound that does not contain both the structures represented by the above formula (A) and the above formula (B) (hereinafter, also referred to as a "second chain-like fluorine compound").

[0068] The carbon number of the first chain fluorine compound is not particularly limited, but is, for example, 3 or more and 15 or less. From the viewpoint of further improving the solubility of the electrolyte in the electrolytic solution, the carbon number of the first chain fluorine compound is preferably 4 or more, 5 or more, or 6 or more. From the same viewpoint, the carbon number of the first chain fluorine compound is preferably 14 or less, 12 or less, 10 or less, or 8 or less.

[0069] The number of carbon atoms in the second chain fluorine compound is not particularly limited, but is, for example, 3 or more and 20 or less. From the viewpoint of further improving the solubility of the electrolyte in the electrolytic solution, the number of carbon atoms in the second chain fluorine compound is preferably 4 or more, 5 or more, 6 or more, 7 or more, 8 or more, or 10 or more. From the same viewpoint, the number of carbon atoms in the second chain fluorine compound is preferably 18 or less, 15 or less, or 12 or less.

[0070] In this embodiment, the first chain fluorine compound is not particularly limited as long as it is a compound having a monovalent group represented by the above formula (A) or formula (B), and examples thereof include a compound having an ether bond, a compound having an ester bond, and a compound having a carbonate bond, etc. From the viewpoint of further improving the solubility of the electrolyte in the electrolytic solution and further improving the cycle characteristics of the battery, the first chain fluorine compound is preferably an ether compound having an ether bond.

[0071] The first chain fluorine compound, which is an ether compound, is not particularly limited. Examples of the compound containing both the structure represented by the above formula (A) and the above formula (B) include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,1,2,2-tetrafluoroethoxy-2,2,3,3-tetrafluoropropoxymethane. Furthermore, examples of compounds containing a structure represented by the above formula (A) and not containing a structure represented by the above formula (B) include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl-1,1,2,2-tetrafluoroethyl ether, ethyl-1,1,2,2-tetrafluoroethyl ether, and propyl-1,1,2,2-tetrafluoroethyl ether. Furthermore, examples of the compound that does not contain a structure represented by the above formula (A) and contains a structure represented by the above formula (B) include difluoromethyl-2,2,3,3-tetrafluoropropyl ether, trifluoromethyl-2,2,3,3-tetrafluoropropyl ether, and difluoromethyl-2,2,3,3-tetrafluoropropyl ether. From the viewpoint of improving the cycle characteristics and / or rate characteristics of the lithium secondary battery, the first chain fluorine compound is preferably selected from 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

[0072] The second chain fluorine compound is not particularly limited, but examples thereof include methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether, 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-trifluoromethylpentane, methyl-2,2,3,3,3-pentafluoropropyl ether, and 1,1,2,3,3,3-hexafluoropropyl methyl ether.

[0073] When the electrolytic solution contains a fluorine compound other than a cyclic fluorine compound, the content of the cyclic fluorine compound is not particularly limited, and may be 10 vol. % or more, 15 vol. % or more, 20 vol. % or more, or 25 vol. % or more, and may be 90 vol. % or less, 80 vol. % or less, 70 vol. % or less, or 50 vol. % or less, relative to the total amount of the fluorine compounds. When the electrolytic solution contains a first chain fluorine compound, the content of the first chain fluorine compound is not particularly limited, and may be 50 volume % or more, 60 volume % or more, or 70 volume % or more, and may be 95 volume % or less, 90 volume % or less, or 85 volume % or less, relative to the total amount of the fluorine compounds. When the electrolytic solution contains a second chain fluorine compound, the content of the second chain fluorine compound is not particularly limited, and may be 5 vol% or more, or 10 vol% or more, and may be 50 vol% or less, 40 vol% or less, 30 vol% or less, 20 vol% or less, or 15 vol% or less, relative to the total amount of the fluorine compounds. When the electrolyte contains two or more kinds of cyclic fluorine compounds, the total amount of the cyclic fluorine compounds is defined as the content of the cyclic fluorine compounds. The same applies when the electrolyte contains two or more kinds of first chain fluorine compounds or second chain fluorine compounds.

[0074] The content of the first chain fluorine compound in the electrolytic solution is not particularly limited, but is, for example, 0% by volume or more and 90% by volume or less with respect to the total amount of the solvent components of the electrolytic solution.The content of the first chain fluorine compound is preferably 5% by volume or more, 10% by volume or more, 15% by volume or more, 20% by volume or more, 30% by volume or more, or 40% by volume or more.The content of the first chain fluorine compound is preferably 85% by volume or less, 80% by volume or less, 75% by volume or less, 70% by volume or less, 65% by volume or less, 60% by volume or less, or 50% by volume or less.

[0075] In this embodiment, the electrolytic solution preferably contains an ether compound having no fluorine atoms (hereinafter referred to as "ether co-solvent") as a solvent. According to this embodiment, the solubility of the electrolyte in the electrolytic solution is further improved, so that the ionic conductivity of the electrolytic solution is improved, and as a result, the lithium secondary battery 100 has excellent cycle characteristics. The ether co-solvent is not particularly limited as long as it is a compound having no fluorine atoms and an ether bond. In the usage environment of the lithium secondary battery, it is preferable that the ether co-solvent can dissolve the electrolyte to prepare an electrolytic solution in a solution phase, or that the compound alone or a mixture with other compounds is liquid.

[0076] The number of carbon atoms in the ether co-solvent is not particularly limited and is, for example, 2 or more and 20 or less. From the viewpoint of further improving the cycle characteristics of the battery, the number of carbon atoms in the ether co-solvent is preferably 3 or more, or 4 or more, and may be 5 or more, or 6 or more. From the same viewpoint, the number of carbon atoms in the ether co-solvent is preferably 15 or less, 12 or less, 10 or less, 9 or less, or 7 or less.

[0077] The number of ether bonds in the ether co-solvent is not particularly limited, but is, for example, 1 or more and 10 or less. From the viewpoint of further improving the cycle characteristics of the battery, the ether co-solvent preferably has 2 or more ether bonds, and may have 3, 4, 5, 6, 7, or 8 ether bonds. The number of ether bonds is preferably 2 or more and 5 or less.

[0078] The ether co-solvent is not particularly limited as long as it is an ether compound having no fluorine atom, and examples thereof include triethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,3-dimethoxypropane, 1,4-dimethoxybutane, 1,1-dimethoxyethane, 1,2-dimethoxypropane, 2,2-dimethoxypropane, 1,3-dimethoxybutane, 1,2-dimethoxybutane, 2,2-dimethoxybutane, 2,3-dimethoxybutane, 1,2-diethoxypropane, 1,2-diethoxybutane, 2,3-diethoxybutane, and diethoxyethane. From the viewpoint of further improving the cycle characteristics of the battery, 1,2-dimethoxyethane, 1,2-dimethoxypropane, and 2,2-dimethoxypropane are preferred as the ether co-solvent.

[0079] The content of the ether sub-solvent in the electrolyte of this embodiment is not particularly limited, but is, for example, 0% by volume or more and 80% by volume or less with respect to the total amount of the solvent components of the electrolyte. From the viewpoint of effectively and reliably achieving the above-mentioned effects of the cyclic fluorine compound in the lithium secondary battery, the content of the cyclic fluorine compound is preferably 5.0% by volume or more, 10% by volume or more, 15% by volume or more, or 20% by volume or more with respect to the total amount of the solvent components of the electrolyte. From the same viewpoint, the content of the cyclic fluorine compound is preferably 75% by volume or less, 70% by volume or less, 65% by volume or less, 60% by volume or less, or 55% by volume or less with respect to the total amount of the solvent components of the electrolyte.

[0080] The electrolytic solution may further contain a compound having no fluorine atom other than the above-mentioned ether co-solvent (hereinafter also referred to as "non-ether co-solvent") as a solvent. The non-ether co-solvent is not particularly limited, and examples thereof include acetonitrile, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, and triethyl phosphate. The non-ether co-solvent may have at least one group selected from the group consisting of a carbonate group, a carbonyl group, a ketone group, and an ester group.

[0081] The electrolyte solution may contain at least one cyclic fluorine compound. From the viewpoint of further improving the solubility of the electrolyte in the electrolyte solution and further improving the cycle characteristics of the battery, the electrolyte solution preferably contains two or more fluorine compounds. From the same viewpoint, the electrolyte solution preferably contains at least one cyclic fluorine compound and at least one first chain fluorine compound.

[0082] As the solvent of the electrolytic solution, the above-mentioned cyclic fluorine compound, the above-mentioned first chain fluorine compound, the above-mentioned second chain fluorine compound, the above-mentioned ether sub-solvent and the above-mentioned non-ether sub-solvent can be used in any combination. In addition, for each solvent, each solvent may be used alone or in combination of two or more. In the electrolytic solution, the compound not having a fluorine atom may be used alone as an ether sub-solvent, or in combination with a non-ether sub-solvent.

[0083] The structural formulas of compounds that may be contained as a solvent in this embodiment are shown in the following table. The cyclic fluorine compounds are shown in Table 1. The first chain fluorine compounds and the second chain fluorine compounds are shown in Table 2. Furthermore, ether co-solvents are shown in Table 3. However, the types of compounds that can be used as a solvent are not limited to these.

[0084] [Table 1]

[0085] [Table 2]

[0086] [Table 3]

[0087] The lithium salt contained in the electrolyte is not particularly limited, but may be an inorganic salt or an organic salt of lithium. Specifically, LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiBF2(C2O4), LiB(O2C2H4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4 may be mentioned. From the viewpoint of further improving the energy density and cycle characteristics of the lithium secondary battery 100, LiN(SO2F)2 is preferred as the lithium salt. The above lithium salts may be used alone or in combination of two or more. The electrolytic solution may further contain a salt other than the lithium salt as an electrolyte, such as, for example, salts of Na, K, Ca, and Mg.

[0088] The concentration of the lithium salt in the electrolyte is not particularly limited, but is preferably 0.5M or more, more preferably 0.7M or more, even more preferably 0.9M or more, and even more preferably 1.0M or more. When the concentration of the lithium salt is within the above range, the SEI layer is more easily formed, and the internal resistance tends to be further lower. In particular, the lithium secondary battery 100 containing a fluorine compound as a solvent can increase the concentration of the lithium salt in the electrolyte, so that the cycle characteristics and rate performance can be further improved. The upper limit of the concentration of the lithium salt is not particularly limited, and the concentration of the lithium salt may be 10.0M or less, 5.0M or less, or 2.0M or less.

[0089] The lithium secondary battery of this embodiment may contain an electrolytic solution or an electrolytic solution component in a state other than a liquid. For example, by adding an electrolytic solution when preparing a separator described later, a battery containing the electrolytic solution in a solid or semi-solid (gel) material can be obtained. The electrolytic solution can also be referred to as an electrolyte.

[0090] The presence of the cyclic fluorine compound and the ether co-solvent in the electrolyte can be confirmed by various conventional methods, such as NMR measurement, mass spectrometry such as HPLC-MS, and IR measurement.

[0091] The molecular structure of the solvent contained in the electrolyte can be estimated by measuring or analyzing it by a known method. Such methods include, for example, NMR, mass spectrometry, elemental analysis, and infrared spectroscopy. The molecular structure of the solvent can also be estimated by theoretical calculation using molecular dynamics, molecular orbital, and the like.

[0092] (Solid electrolyte interface layer) In the lithium secondary battery 100, it is presumed that a solid electrolyte interface layer (SEI layer) is formed on the surface of the negative electrode 140 by charging, particularly initial charging, but the lithium secondary battery 100 does not need to have an SEI layer. The SEI layer formed is presumed to contain an organic compound derived from the above-mentioned cyclic fluorine compound and lithium salt, but may also contain, for example, other inorganic compounds containing lithium and / or organic compounds containing lithium.

[0093] The organic compound containing lithium and the inorganic compound containing lithium are not particularly limited as long as they are contained in a conventionally known SEI layer. Although not intended to be limiting, the organic compound containing lithium includes organic compounds such as alkyl lithium carbonate, lithium alkoxide, and lithium alkyl ester, and the inorganic compound containing lithium includes LiF, Li2CO3, Li2O, LiOH, lithium borate compound, lithium phosphate compound, lithium sulfate compound, lithium nitrate compound, lithium nitrite compound, and lithium sulfite compound.

[0094] The lithium secondary battery 100 contains a cyclic fluorine compound as a solvent and may further contain a chain-like fluorine compound, which is presumably why the formation of the SEI layer is promoted. Since the SEI layer has ion conductivity, the reactivity of the lithium deposition reaction on the negative electrode surface on which the SEI layer is formed is uniform in the planar direction of the negative electrode surface. Therefore, the lithium secondary battery 100 is considered to have excellent cycle characteristics because the growth of dendritic lithium metal on the negative electrode is suppressed.

[0095] A typical average thickness of the SEI layer is 1 nm or more and 10 μm or less. When the SEI layer is formed in the lithium secondary battery 100, lithium metal precipitated by charging the battery may precipitate at the interface between the negative electrode 140 and the SEI layer, or may precipitate at the interface between the SEI layer and the separator.

[0096] (positive electrode) The positive electrode 120 is not particularly limited as long as it is generally used in lithium secondary batteries, but a known material can be appropriately selected depending on the application of the lithium secondary battery. From the viewpoint of increasing the stability and output voltage of the lithium secondary battery 100, the positive electrode 120 preferably has a positive electrode active material.

[0097] In this specification, the term "positive electrode active material" refers to a material for holding a lithium element (typically, lithium ion) in a positive electrode in a battery, and may be referred to as a host material for a lithium element (typically, lithium ion). Such positive electrode active materials are not particularly limited, but include, for example, metal oxides and metal phosphates. The metal oxides are not particularly limited, but include, for example, cobalt oxide-based compounds, manganese oxide-based compounds, and nickel oxide-based compounds. The metal phosphates are not particularly limited, but include, for example, iron phosphate-based compounds, and cobalt phosphate-based compounds. Typical positive electrode active materials include LiCoO2, LiNi x Co y Mn z O2(x+y+z=1), LiNi x Co y Alz O(x+y+z=1), LiNi x Mn y O2 (x+y=1), LiNiO2, LiMn2O4, LiFePO4, LiCoPO4, FeF3, LiFeOF, LiNiOF, and TiS2.

[0098] The above-mentioned positive electrode active materials are used alone or in combination of two or more. The positive electrode 120 may contain components other than the above-mentioned positive electrode active materials. Such components are not particularly limited, but may include, for example, a conductive assistant, a binder, and an electrolyte.

[0099] The conductive assistant in the positive electrode 120 is not particularly limited, but examples thereof include carbon black, single-walled carbon nanotubes (SW-CNT), multi-walled carbon nanotubes (MW-CNT), carbon nanofibers, and acetylene black. The binder is not particularly limited, but examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene butadiene rubber, acrylic resin, and polyimide resin. Examples of the electrolyte in the positive electrode 120 include a polymer electrolyte, a gel electrolyte, and an inorganic solid electrolyte, and a typical electrolyte is a polymer electrolyte or a gel electrolyte. As the polymer electrolyte and the gel electrolyte, those described below can be used. The conductive assistant, binder, and electrolyte as described above may be used alone or in combination of two or more.

[0100] The content of the positive electrode active material in the positive electrode 120 may be, for example, 50% by mass or more and 100% by mass or less with respect to the entire positive electrode 120. The content of the conductive assistant may be, for example, 0.5% by mass or more and 30% by mass or less with respect to the entire positive electrode 120. The content of the binder may be, for example, 0.5% by mass or more and 30% by mass or less with respect to the entire positive electrode 120. The total content of the electrolyte may be, for example, 0.5% by mass or more and 30% by mass or less with respect to the entire positive electrode 120.

[0101] The average thickness of the positive electrode 120 is, for example, 10 μm to 300 μm, preferably 30 μm to 200 μm, or 50 μm to 150 μm, although the average thickness of the positive electrode can be appropriately adjusted depending on the desired battery capacity.

[0102] (Positive electrode current collector) A positive electrode current collector 110 is formed on one side of the positive electrode 120. There are no particular limitations on the positive electrode current collector 110 as long as it is a conductor that does not react with lithium ions in the battery. Examples of such positive electrode current collectors include aluminum.

[0103] The average thickness of the positive electrode current collector 110 of this embodiment is preferably 4 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and further preferably 6 μm or more and 15 μm or less. According to such an embodiment, the volume occupied by the positive electrode current collector 110 in the lithium secondary battery 100 is reduced, and therefore the energy density of the lithium secondary battery 100 is further improved.

[0104] (Separator) The separator 130 of this embodiment is a member for preventing the battery from short-circuiting by isolating the positive electrode 120 and the negative electrode 140, while ensuring the ionic conductivity of lithium ions that serve as charge carriers between the positive electrode 120 and the negative electrode 140. That is, the separator 130 has a function of isolating the positive electrode 120 and the negative electrode 140, and a function of ensuring the ionic conductivity of lithium ions. The separator 130 also plays a role in retaining the electrolyte. As such a separator, one type of member having the above two functions may be used alone, or two or more types of members having one of the above functions may be used in combination. The separator is not particularly limited as long as it has the above-mentioned function, but examples thereof include insulating porous members, polymer electrolytes, gel electrolytes, and inorganic solid electrolytes, and is typically at least one selected from the group consisting of insulating porous members, polymer electrolytes, and gel electrolytes.

[0105] When the separator 130 includes an insulating porous member, the pores of the member are filled with a substance having ion conductivity, so that the member exhibits ion conductivity. Examples of the substance to be filled include the above-mentioned electrolytic solution, polymer electrolyte, and gel electrolyte. For the separator 130, a porous member having insulating properties, a polymer electrolyte, or a gel electrolyte can be used alone or in combination of two or more of them.

[0106] The material constituting the insulating porous member is not particularly limited, but may be, for example, an insulating polymer material, specifically, polyethylene (PE) and polypropylene (PP). That is, the separator 130 may be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminated structure thereof.

[0107] The polymer electrolyte is not particularly limited, but examples thereof include solid polymer electrolytes mainly containing a polymer and an electrolyte, and semi-solid polymer electrolytes mainly containing a polymer, an electrolyte, and a plasticizer. The gel electrolyte is not particularly limited, but may be, for example, one that mainly contains a polymer and a liquid electrolyte (that is, a solvent and an electrolyte).

[0108] Polymers that may be included in the polymer electrolyte and gel electrolyte include, but are not limited to, polymers containing functional groups containing oxygen atoms such as ether and ester, halogen groups, and polar groups such as cyano groups.Specific examples include resins having ethylene oxide units in the main chain and / or side chain such as polyethylene oxide (PEO), resins having propylene oxide units in the main chain and / or side chain such as polypropylene oxide (PPO), acrylic resins, vinyl resins, ester resins, nylon resins, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polysiloxane, polyphosphazene, polymethyl methacrylate, polyamide, polyimide, aramid, polylactic acid, polyurethane, polyacetal, polysulfone, polyethylene carbonate, polypropylene carbonate, and polytetrafluoroethylene.The above-mentioned resins may be used alone or in combination of two or more.

[0109] Examples of electrolytes contained in the polymer electrolyte and the gel electrolyte include salts of Li, Na, K, Ca, and Mg. Typically, in this embodiment, the polymer electrolyte and the gel electrolyte contain a lithium salt. The lithium salt is not particularly limited, but examples thereof include LiI, LiCl, LiBr, LiF, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2CF3CF3)2, LiB(O2C2H4)2, LiB(C2O4)2, LiB(O2C2H4)F2, LiB(OCOCF3)4, LiNO3, and Li2SO4, and is preferably at least one selected from the group consisting of LiN(SO2F)2, LiN(SO2CF3)2, and LiN(SO2CF3CF3)2. The above-mentioned salts or lithium salts are used alone or in combination of two or more.

[0110] The blending ratio of the polymer and the lithium salt in the polymer electrolyte and the gel electrolyte may be determined by the ratio of the polar group of the polymer to the lithium atom of the lithium salt. For example, when the polymer has oxygen atoms, the blending ratio may be determined by the ratio ([Li] / [O]) of the number of oxygen atoms of the polymer to the number of lithium atoms of the lithium salt. In the polymer electrolyte and the gel electrolyte, the blending ratio of the polymer and the lithium salt may be adjusted so that the ratio ([Li] / [O]) is, for example, 0.02 to 0.20, 0.03 to 0.15, or 0.04 to 0.12.

[0111] The solvent contained in the gel electrolyte is not particularly limited, but for example, the solvents that can be contained in the above-mentioned electrolytic solution can be used alone or in combination of two or more. Preferred examples of the solvent are the same as those in the electrolytic solution described below. The plasticizer contained in the semi-solid polymer electrolyte is not particularly limited, but may include, for example, components similar to the solvent that may be contained in the gel electrolyte, and various oligomers.

[0112] The separator 130 may be coated with a separator coating layer. The separator coating layer may cover both sides of the separator 130, or may cover only one side. The separator coating layer is not particularly limited as long as it is a member that does not react with lithium ions, but it is preferable that the separator coating layer is one that can firmly bond the separator 130 and a layer adjacent to the separator 130. Examples of such a separator coating layer include, but are not particularly limited to, polyvinylidene fluoride (PVDF), a mixture of styrene butadiene rubber and carboxymethyl cellulose (SBR-CMC), polyacrylic acid (PAA), lithium polyacrylate (Li-PAA), polyimide (PI), polyamideimide (PAI), and a binder such as aramid. The separator coating layer may be formed by adding inorganic particles such as silica, alumina, titania, zirconia, magnesium oxide, magnesium hydroxide, and lithium nitrate to the binder.

[0113] The average thickness of the separator 130 is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. According to such an embodiment, the volume occupied by the separator 130 in the lithium secondary battery 100 is reduced, and the energy density of the lithium secondary battery 100 is further improved. Moreover, the average thickness of the separator 130 is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. According to such an embodiment, the positive electrode 120 and the negative electrode 140 can be more reliably isolated from each other, and short-circuiting of the battery can be further suppressed.

[0114] (Use of lithium secondary batteries) 2 shows one mode of use of the lithium secondary battery of this embodiment. In the lithium secondary battery 200, a positive electrode terminal 210 and a negative electrode terminal 220 for connecting the lithium secondary battery 200 to an external circuit are joined to a positive electrode current collector 110 and a negative electrode 140, respectively. The lithium secondary battery 200 is charged and discharged by connecting the negative electrode terminal 220 to one end of the external circuit and the positive electrode terminal 210 to the other end of the external circuit.

[0115] The lithium secondary battery 200 is charged by applying a voltage between the positive electrode terminal 210 and the negative electrode terminal 220 such that a current flows from the negative electrode terminal 220 (negative electrode 140) through an external circuit to the positive electrode terminal 210 (positive electrode 120). It is presumed that the lithium secondary battery 200 forms a solid electrolyte interface layer (SEI layer) on the surface of the negative electrode 140 (interface between the negative electrode 140 and the separator 130) by initial charging, but the lithium secondary battery 200 does not need to have an SEI layer. Charging the lithium secondary battery 200 causes deposition of lithium metal at the interface between the negative electrode 140 and the SEI layer, the interface between the negative electrode 140 and the separator 130, and / or the interface between the SEI layer and the separator 130.

[0116] When the positive electrode terminal 210 and the negative electrode terminal 220 of the charged lithium secondary battery 200 are connected via a desired external circuit, the lithium secondary battery 200 is discharged. This causes the deposition of lithium metal on the negative electrode to be electrolytically dissolved. When an SEI layer is formed in the lithium secondary battery 200, the deposition of lithium metal on at least one of the interface between the negative electrode 140 and the SEI layer and / or the interface between the SEI layer and the separator 130 is electrolytically dissolved.

[0117] (Lithium secondary battery manufacturing method) There are no particular limitations on the method for manufacturing the lithium secondary battery 100 as shown in FIG. 1, so long as it is a method capable of manufacturing a lithium secondary battery having the above-mentioned configuration, and examples thereof include the following methods.

[0118] The positive electrode current collector 110 and the positive electrode 120 are manufactured, for example, as follows. The above-mentioned positive electrode active material, conductive assistant, and binder are mixed to obtain a positive electrode mixture. The compounding ratio may be, for example, 50% by mass or more and 99% by mass or less of the positive electrode active material, 0.5% by mass or more and 30% by mass or less of the conductive assistant, and 0.5% by mass or more and 30% by mass or less of the binder, relative to the entire positive electrode mixture. The obtained positive electrode mixture is applied to one side of a metal foil (e.g., Al foil) having a predetermined thickness (e.g., 5 μm to 1 mm) as a positive electrode current collector, and press molded. The obtained molded body is punched out to a predetermined size by a punching process to obtain the positive electrode current collector 110 and the positive electrode 120.

[0119] Next, the above-mentioned negative electrode material, for example, a metal foil (for example, electrolytic Cu foil) having a thickness of 1 μm or more and 1 mm or less, is washed with a solvent containing sulfamic acid, punched out to a predetermined size, and then ultrasonically cleaned with ethanol and dried to obtain the negative electrode 140.

[0120] Next, the separator 130 having the above-mentioned configuration is prepared. The separator 130 may be manufactured by a conventionally known method, or a commercially available separator may be used.

[0121] Next, at least one of the cyclic fluorine compounds and, if necessary, the ether co-solvent, the chain fluorine compound, and / or the non-ether co-solvent are mixed to obtain a solution, and a lithium salt is dissolved in the solution to prepare an electrolyte solution. The mixing ratio of the solvent and the lithium salt may be appropriately adjusted so that the content or concentration of each solvent and the lithium salt in the electrolyte solution falls within the above-mentioned range.

[0122] The positive electrode current collector 110 having the positive electrode 120 formed thereon, the separator 130, and the negative electrode 140 are laminated in this order so that the positive electrode 120 faces the separator 130 to obtain a laminate. The obtained laminate is enclosed in a sealed container together with an electrolyte solution to obtain a lithium secondary battery 100. The sealed container is not particularly limited, but may be, for example, a laminate film.

[0123] [Variations] The above-described embodiment is an example for explaining the present invention, and is not intended to limit the present invention to only this embodiment. The present invention can be modified in various ways without departing from the gist of the invention.

[0124] For example, the separator 130 may be omitted in the lithium secondary battery 100. In that case, it is preferable to fix the positive electrode 120 and the negative electrode 140 in a state where they are sufficiently separated from each other so that they are not in physical or electrical contact with each other.

[0125] The lithium secondary battery of this embodiment may have a current collector disposed on the surface of the negative electrode so as to contact the negative electrode. Such a current collector is not particularly limited, but may be, for example, a material that can be used for the negative electrode. When the lithium secondary battery does not have a positive electrode current collector or a negative electrode current collector, the positive electrode or the negative electrode itself acts as a current collector, respectively.

[0126] In the lithium secondary battery of this embodiment, a part or the whole of the surface of the negative electrode facing the separator may be coated with a coating agent. Examples of the negative electrode coating agent include benzotriazole (BTA), imidazole (IM), and triazinethiol (TAS), as well as derivatives thereof. For example, the above-mentioned negative electrode material can be washed, immersed in a solution containing the negative electrode coating agent (for example, a solution containing 0.01% by volume or more and 10% by volume or less of the negative electrode coating agent), and then dried under air to coat the negative electrode coating agent. It is presumed that the coating of the above-mentioned compound suppresses uneven deposition of lithium metal on the surface of the negative electrode, and suppresses the lithium metal deposited on the negative electrode from growing in a dendrite shape.

[0127] In the lithium secondary battery of this embodiment, a terminal for connecting to an external circuit may be attached to the positive electrode current collector and / or the negative electrode. For example, a metal terminal (e.g., Al, Ni, etc.) having a thickness of 10 μm to 1 mm may be bonded to one or both of the positive electrode current collector and the negative electrode. As a bonding method, a conventionally known method may be used, for example, ultrasonic welding may be used.

[0128] In this specification, "high energy density" or "having a high energy density" means that the capacity per unit total volume or total mass of the battery is high, and is preferably 700 Wh / L or more or 300 Wh / kg or more, more preferably 800 Wh / L or more or 350 Wh / kg or more, and even more preferably 900 Wh / L or more or 400 Wh / kg or more.

[0129] In addition, in this specification, "having excellent cycle characteristics" means that the capacity of the battery is reduced at a low rate before and after a number of charge / discharge cycles that can be expected in normal use. That is, when comparing the first discharge capacity after the initial charge / discharge and the capacity after a number of charge / discharge cycles that can be expected in normal use, the capacity after the charge / discharge cycle is almost not reduced compared to the first discharge capacity after the initial charge / discharge. Here, the "number of times that can be expected in normal use" is, for example, 30 times, 50 times, 70 times, 100 times, 300 times, or 500 times, depending on the application of the lithium secondary battery. In addition, "the capacity after the charge / discharge cycle is almost not reduced compared to the first discharge capacity after the initial charge / discharge" means, for example, that the capacity after the charge / discharge cycle is 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, or 85% or more compared to the first discharge capacity after the initial charge / discharge, depending on the application of the lithium secondary battery.

[0130] In this specification, the numerical ranges described as preferred ranges etc. may be replaced with numerical ranges obtained by combining any of the upper and lower limit values ​​described. For example, when a certain parameter is preferably 50 or more, more preferably 60 or more, and preferably 100 or less, more preferably 90 or less, the parameter may be any of 50 or more and 100 or less, 50 or more and 90 or less, 60 or more and 100 or less, or 60 or more and 90 or less. EXAMPLES

[0131] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following test examples.

[0132] [Example 1] The lithium secondary battery of Example 1 was fabricated as follows.

[0133] (Preparation of the negative electrode) First, 8 μm electrolytic Cu foil was washed with a solvent containing sulfamic acid, then punched out to a specified size (45 mm × 45 mm). Further, it was ultrasonically cleaned with ethanol, and then dried to obtain Cu foil, which was used as the negative electrode.

[0134] (Preparation of positive electrode) Next, the positive electrode was prepared. LiNi was used as the positive electrode active material. 0.85 Co 0.12 Al 0.03 A mixture of 96 parts by mass of O2, 2 parts by mass of carbon black as a conductive assistant, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder was applied to one side of a 12 μm Al foil and press molded. The obtained molded body was punched out to a specified size (40 mm × 40 mm) by punching to obtain a positive electrode having a positive electrode current collector on one side.

[0135] (Preparation of separator) A separator of a given size (50 mm×50 mm) was prepared, in which both sides of a 12 μm polyethylene microporous membrane were coated with 2 μm polyvinylidene fluoride (PVDF).

[0136] (Preparation of electrolyte) The electrolyte was prepared as follows: 1,1,2,2,3,3,4-heptafluorocyclopentane and 1,2-dimethoxyethane were mixed so that the volume percentages were 80% and 20%, respectively. LiN(SO2F)2 was dissolved in the resulting mixture so that the molar concentration was 1.0M, to obtain an electrolyte.

[0137] (Battery Assembly) The positive electrode current collector on which the positive electrode was formed, the separator, and the negative electrode were stacked in this order so that the positive electrode faced the separator to obtain a laminate. Furthermore, a 100 μm Al terminal and a 100 μm Ni terminal were joined to the positive electrode current collector and the negative electrode, respectively, by ultrasonic welding, and then inserted into a laminate exterior body. Next, the electrolyte solution obtained as described above was injected into the exterior body. The exterior body was sealed to obtain a lithium secondary battery.

[0138] [Examples 2 to 14] A lithium secondary battery was obtained in the same manner as in Example 1, except that the electrolyte solution was prepared using the solvent shown in Table 4.

[0139] [Comparative Examples and Reference Comparative Examples] A lithium secondary battery was obtained in the same manner as in Example 1, except that the electrolyte solution was prepared using the solvents shown in Table 4. The comparative example does not contain a cyclic fluorine compound and contains only an ether co-solvent, and the reference comparative example contains an ether co-solvent and a chain fluorine compound.

[0140] In Table 4, "HFCP" stands for 1,1,2,2,3,3,4-heptafluorocyclopentane, "HDFMCH" for hexadecafluoro(1,3-dimethylcyclohexane), "TDFMCH" for tetradecafluoromethylcyclohexane, "ODFDHN" for octadecafluorodecahydronaphthalene, and "FCP" for fluorocyclopentane. "TTFE" stands for 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and "TFEE" stands for 1,1,2,2-tetrafluoroethyl-2,2. Furthermore, "DME" stands for 1,2-dimethoxyethane, and "DMP" stands for 1,2-dimethoxypropane.

[0141] In Table 4, each solvent is classified as either a cyclic fluorine compound, a chain fluorine compound, or an ether co-solvent, as defined above. In addition, in Table 4, the content of each solvent relative to the total amount of the solvent is listed in volume percent in the column to the right of each solvent. For example, in Table 4, Example 1 means that it contains 80 volume percent HFCP and 20 volume percent DME. All of the Examples, Comparative Examples, and Reference Comparative Examples contain 1.0 M LiN(SO2F)2 as an electrolyte.

[0142] [Evaluation of cycle characteristics] The cycle characteristics of the lithium secondary batteries prepared in each of the Examples and Comparative Examples were evaluated as follows.

[0143] The lithium secondary batteries thus fabricated were CC-charged at 3.2 mA until the voltage reached 4.2 V (initial charge), and then CC-discharged at 3.2 mA until the voltage reached 3.0 V (hereinafter referred to as "initial discharge"). Next, the batteries were CC-charged at 13.6 mA until the voltage reached 4.2 V, and then CC-discharged at 20.4 mA until the voltage reached 3.0 V, and this cycle was repeated in an environment at a temperature of 25° C. For each example, the capacity determined from the initial discharge (hereinafter referred to as "initial capacity" and referred to as "capacity" in the table) is shown in Table 4. For each example, the number of cycles at which the discharge capacity reached 80% of the initial capacity (referred to as "cycle" in the table) is also shown in Table 4.

[0144] [Volume expansion rate measurement] The volume expansion rate of the lithium secondary batteries produced in each of the Examples and Comparative Examples was evaluated as follows. For each example, the thickness of the cell immediately after fabrication and the thickness of the cell after 99 charge / discharge cycles and the 100th charge were measured using a microgauge to determine the volume expansion rate associated with charge / discharge. The volume expansion rate of the cell after the 100th charge compared to the cell immediately after fabrication (meaning the expansion ratio of the cell after the 100th charge compared to the thickness of the cell immediately after fabrication) is shown in Table 4 as "Volume Expansion Rate (%)".

[0145] [Table 4]

[0146] In Table 4, "-" means that the corresponding component is not present, and "NA" means that the volume expansion rate was not measured.

[0147] From Table 4, it can be seen that Examples 1 to 14, which use an electrolyte solution containing the above-mentioned cyclic fluorine compound, have superior cycle characteristics compared to Comparative Examples, which do not. In addition, when comparing the Reference Comparative Example with Examples 1 to 4, it can be seen that Examples 1 to 4 have low volume expansion coefficients, and the lithium secondary battery of this embodiment has even superior safety. INDUSTRIAL APPLICABILITY

[0148] INDUSTRIAL APPLICABILITY The lithium secondary battery of the present invention has high energy density and excellent cycle characteristics, and therefore has industrial applicability as an electricity storage device for various applications. [Explanation of symbols]

[0149] 100, 200...lithium secondary battery, 110...positive electrode current collector, 120...positive electrode, 130...separator, 140...negative electrode, 210...positive electrode terminal, 220...negative electrode terminal.

Claims

1. A battery comprising a positive electrode, a negative electrode having no negative electrode active material, and an electrolyte; The electrolyte solution is A lithium salt; and a cyclic fluorine compound having a cyclic hydrocarbon skeleton substituted with at least one fluorine atom. Lithium secondary battery.

2. 2. The lithium secondary battery according to claim 1, wherein the cyclic fluorine compound has polarity.

3. 3. The lithium secondary battery according to claim 1, wherein in the cyclic fluorine compound, a ratio (F / (F+H)) of the number of fluorine atoms (F) to the total number (F+H) of fluorine atoms and hydrogen atoms is 0.20 or more and 1.0 or less.

4. 4. The lithium secondary battery according to claim 1, wherein the number of carbon atoms constituting the cyclic hydrocarbon skeleton is 4 or more and 15 or less.

5. The electrolyte is LiN(SO ２ F) ２ The lithium secondary battery according to any one of claims 1 to 4, comprising as a lithium salt.

6. 6. The lithium secondary battery according to claim 1, wherein the content of the cyclic fluorine compound is 10% by volume or more and 90% by volume or less with respect to the total amount of the solvent components of the electrolyte solution.

7. 7. The lithium secondary battery according to claim 1, wherein the electrolyte further contains an ether compound having no fluorine atoms.

8. 8. The lithium secondary battery according to claim 7, wherein the ether compound is a compound containing 2 to 5 ether bonds.

9. 9. The lithium secondary battery according to claim 7, wherein the content of the ether compound is 10% by volume or more and 70% by volume or less with respect to the total amount of the solvent components of the electrolyte solution.

10. The electrolyte solution further contains a chain fluorine compound having at least one of the monovalent groups represented by the following formula (A) or formula (B): The lithium secondary battery according to any one of claims 1 to 9. 【Chemistry 1】 【Chemistry 2】 (In the above formula, the wavy line represents a bonding site in a monovalent group.)

11. 11. The lithium secondary battery according to claim 10, wherein the content of the chain fluorine compound is 10% by volume or more and 85% by volume or less with respect to the total amount of the solvent components of the electrolyte solution.

12. 12. The lithium secondary battery according to claim 1, wherein the cyclic hydrocarbon skeleton is a saturated cyclic hydrocarbon skeleton.

13. In the cyclic fluorine compound, a ratio (F / (F+H)) of the number of fluorine atoms (F) to the total number of fluorine atoms and hydrogen atoms (F+H) is 0.7 or more and 1.0 or less, 13. The lithium secondary battery according to claim 1, wherein the number of carbon atoms constituting the cyclic hydrocarbon skeleton is 5 or 6.

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